Reverse osmosis pushes household water against a semipermeable membrane under line pressure. Water molecules pass through, most dissolved solids do not, and the rejected material leaves continuously down the drain. The result is water that is far lower in dissolved content, tastes different, and takes time and some waste to produce.
The membrane is a barrier, not a strainer
A sediment cartridge stops particles by pore size. An RO membrane is a dense film that water diffuses through while dissolved ions are rejected at the surface, which is why it removes things no cartridge can catch. That includes hardness, which the USGS defines as dissolved calcium and magnesium, along with sodium, nitrate, most metals and a wide range of dissolved organics. Rejection is high but never total, and it varies by contaminant.
What comes out and what goes down the drain
Two streams leave the membrane. Permeate is the treated water that fills the storage tank and reaches the faucet. Concentrate carries the rejected material to the drain and keeps the membrane surface swept clean, which is why an RO unit cannot be run without a drain line. The ratio between them depends on line pressure, temperature and membrane design, and low incoming pressure hurts both the ratio and the output.
The taste change and the mineral question
RO water reads clean and slightly flat, because the dissolved minerals that give water body are largely gone. Some households love it, others add a remineralizing post-filter to put a trace of calcium and magnesium back. Nothing about the removal makes the water unsafe: drinking water is a small contributor to mineral intake compared with food. Coffee and tea drinkers often notice the biggest difference, in both directions.
What it does not do
It does not disinfect. A membrane rejects bacteria physically, but any breach, any post-membrane contamination in the tank or tubing, and any bypass through a fitting puts organisms downstream, so RO is not a substitute for treating a well that fails a bacteria test. It also treats one tap, not the house, unless you buy a whole-house unit with storage and a repressurization pump. Our reverse osmosis system guide covers that step up.
Questions people ask about what does reverse osmosis do to water
Does reverse osmosis remove hardness?
Yes, it removes most dissolved calcium and magnesium, but only at the tap it serves. It does not protect the water heater or the rest of the household plumbing.
Why does an RO system send water to the drain?
The concentrate stream carries away the rejected dissolved solids and keeps the membrane surface from fouling. Without it, the membrane would blind off quickly.
Is reverse osmosis water bad for you because minerals are removed?
There is no need to treat it as harmful. If the flat taste bothers you, a remineralization post-filter puts back a trace of calcium and magnesium before the faucet.